Is it conceivable that colonists/astronauts on a long term mission in a low gravity environment might use a low dose of an anabolic steroid (such as Equipoise) to prevent extreme atrophy in their musculature? It's a hypothetical for SF (2250 AD), and a black market dealer is running it out to the colonists on Ganymede and Europa.
Anabolic steroids cause more long term damage to the body's ability to regulate it's own hormones than the value gained in muscle growth. When on steroids your body ceases producing those hormones all together, the longer you are on them the longer it takes your body to begin producing those hormones on its own again. After long enough use your body's ability to regulate its hormone levels can be permanantly damaged. Anabolic steroids also cause damage to major organs like the liver, heart, and kidneys. To put it simply, steroids to combat micro gravity muscle atrophy is like drinking to cure depression.
In the spirit of pure speculation, I would imagine that the side-effects of prolonged steroid use (even low dosage) would be less than ideal due to stress on other internal systems. A better approach might be a form of electrical stimulation similar to what researchers have tried with coma patients. I imagine this tech would be built into clothing to both provide the electrical stimulation and perhaps a degree of resistance to challenge the muscles through natural motion. One might imagine that whatever nano connection these suits maintain with the body could fitted to dispense some form of directed steroid just as it might deliver some form of painkiller in times of stress.
For me, however, the bigger question is why would a drug that is essentially designed to prevent muscle atrophy be distributed as black market contraband? I would imagine that between suits, diet and exercise routines the atrophy issue would be part of daily life (assuming wakefulness and not stasis / hibernation). Black market items would be more about recreationals or something designed to overclock the body (regardless of long term effects).
As already suggested by other users long use of steroids cause permanent harm to the body and its not worth, considering many pro bodybuilders die at age of 25 or so.
A better option would be increasing the nucleai in the muscle cells, this way your astronauts will have an easier time maintaing muscle with exercises.
Nucleus overload is permant and can be obtained either by overtraining muscles with incredibly low rest and high volume at low intensity for months or years or the faster option would be using just one single dose of growth hormones(this is also why steroid users can never be considered natural again even after stoping the use of steroids).